California fires fueled by record drought

An area of disturbed weather has developed near the northern Lesser Antilles Islands, due to a westward-moving tropical wave interacting with an upper-level low. Wind shear is currently too high (30 knots) to allow development of this disturbance. However, the past two runs of the NOGAPS model have predicted that wind shear will fall enough by Friday to allow a tropical depression to form near the western Bahamas. This storm is predicted to move westward across eastern Cuba and into the Western Caribbean. The GFS model does not go along with this scenario, but hints at a weak system developing and moving northeast out sea over the middle Atlantic. The UKMET and ECMWF models do not develop a Bahamas storm, either, but do show a large region of low pressure with low wind shear developing over the Western Caribbean later this week. It would not be a surprise to see a tropical depression develop in the Western Caribbean late this week or early next week.

Southern California's fire storm
Surface maps show a strong high pressure system centered over Nevada and Utah. The clockwise flow of air around this high is driving strong northeasterly winds over Southern California. As the air spills down the mountain passes into coastal San Diego and Los Angeles, gravity helps accelerate the winds. The air compresses and warms as it descends, due to the higher pressures found at sea level. This creates a very hot, low-humidity wind--the dangerous Santa Ana wind. At 1:37 pm PDT yesterday, the humidity in downtown Los Angeles was 8%. Some wind reports Monday afternoon in Southern California, showing the strength of the Santa Ana winds:

Los Angeles County peak wind gusts
------------------------------------------------- --
Leo Carrillo Beach... ... ... ... ... ... ... Northeast 44 mph.
Van Nuys... ... ... ... ... ... ... ... ..... North 39 mph.
Tonner Canyon... ... ... ... ... ... ... .... Northeast 46 mph.
Malibu Canyon... ... ... ... ... ... ... .... Northeast 41 mph.
Malibu Hills... ... ... ... ... ... ... ..... North 54 mph.
Newhall... ... ... ... ... ... ... ... ... .. Northeast 38 mph.
Newhall Pass... ... ... ... ... ... ... ..... North 72 mph.
Saugus... ... ... ... ... ... ... ... ... ... North 60 mph.
Del Valle... ... ... ... ... ... ... ... .... Northeast 50 mph.
Acton... ... ... ... ... ... ... ... ... .... Northeast 45 mph.
Camp Nine... ... ... ... ... ... ... ... .... North 71 mph.
Chilao... ... ... ... ... ... ... ... ... ... Northeast 55 mph.
Mill Creek... ... ... ... ... ... ... ... ... North 41 mph.
Sandberg... ... ... ... ... ... ... ... ..... East 40 mph.
Warm Springs... ... ... ... ... ... ... ..... East 62 mph.
Whitaker Peak... ... ... ... ... ... ... .... North 48 mph.
Lake Palmdale... ... ... ... ... ... ... .... Northeast 46 mph.
Poppy Park... ... ... ... ... ... ... ... ... East 43 mph.
Saddleback Butte... ... ... ... ... ... ..... East 33 mph.
Lancaster... ... ... ... ... ... ... ... .... East 31 mph.

Ventura County peak wind gusts
--------------------------------------------
Oxnard... ... ... ... ... ... ... ... ... ... East 41 mph.
Camarillo... ... ... ... ... ... ... ... .... Northeast 51 mph.
Point Mugu... ... ... ... ... ... ... ... ... Northeast 43 mph.
Piru... ... ... ... ... ... ... ... ... ..... North 49 mph.
Simi Valley... ... ... ... ... ... ... ... .. Northeast 41 mph.
Thousand Oaks... ... ... ... ... ... ... .... North 42 mph.
Laguna Peak... ... ... ... ... ... ... ... .. Northwest 62 mph.

Fourteen large wildfires have developed spanning seven counties in California since Saturday, thanks to the Santa Ana winds, and the prevailing extreme drought conditions. Rainfall in San Diego has been a mere 2.6 inches thus far in 2007, 32% of normal. The July 1, 2006 through June 30, 2007 period was the 4th driest on record in San Diego. In Los Angeles, it was the driest year since record keeping began 130 years ago. Downtown Los Angeles has recorded just 3.37" of rain thus far in 2007--only 28% of normal.

Air quality
Not surprisingly, air quality due to particulate matter has been awful in Southern California (Figure 1). Exceedances of the Federal air quality standards by more than a factor of two have occurred the past three days in both Los Angeles and San Diego. Hightened particulate pollution is strongly correlated with increased death rates, particulary in vulnerable populations, such as those with heart conditions, athsma, or other lung diseases. Everyone should avoid any outdoor exertion; people with respiratory or heart disease, the elderly, and children should remain indoors. Keep your windows and doors closed unless it is extremely hot inside. In these cases, seek alternate shelter. Run your air conditioner if you have one. Keep the fresh air intake closed and the filter clean to prevent bringing additional smoke inside. EPA's Airnow website has more information.


Figure 1. Visible satellite images taken midday on Saturday, Sunday, and Monday. Overlaid on the images is the peak daily particle pollution Air Quality Index (AQI) levels due to smoke. Poor air quality became more widespread across the region as the number and size of the fires increased from day to day. Smoke is visible as white or bluish-white streaks. Blowing dust (brown streaks), reducing visibility under three miles, is also apparent on the images. Image credit: NASA and EPA.

The forecast
The warm, dry, and windy weather will continue today, bringing a repeat of yesterday's extremely dangerous fire situation. A trough of low pressure is expected to move into northern California Wednesday morning, weakening the high pressure system driving the Santa Ana winds. By Thursday, the Santa Ana winds will be gone, and fire fighters will be able to gain the upper hand.

This may only be the first of several serious fire situations in Southern California in the coming months. Santa Ana winds conditions commonly develop during the October through March period, and the extreme drought conditions in Southern California are not going to improve until at least December, when the winter rainy season typically starts. The 3-month precipitation forecast from NOAA's Climate Prediction Center calls for a 33% chance of below-average rainfall over Southern California for the coming winter.

The University of Wisconsin's CIMSS group has a more detailed blog with many weather maps and satellite animations of the Southern California fires.

San Francisco's Climate Challenge
San Francisco residents have a different and more positive kind of challenge this week. An innovative contest designed to encourage residents to reduce energy usage is being launched, with a sign-up deadline of Wednesday. The content offers prizes up to $5000 for those residents able to reduce their energy consumption the most over the coming month. For more information, see the new Weather Underground climate page: at http://www.wunderground.com/climate/. The new page will track current climate trends each month, and feature stories on new research and programs in the climate change field every few weeks. We'll also add a full set of information on the science of climate change over the coming months. The goal is to have a web site that keeps track of the most important issues in climate change.

Jeff Masters